This invention relates to the art of nuclear power plants and it has particular relationship to the transfer of component assemblies of a nuclear reactor during a refueling operation. The expression "refueling operation" as used in this application means an operation during which component assemblies are transferred to and from a reactor for replacement by other assemblies or during which selected component assemblies, for example a fuel assembly, is moved from one position in a reactor to another position in a reactor.
U.S. Pat. No. 4,311,557 granted Jan. 19, 1982 to Edward F. Kowalski, Kenneth J. Swidwa and Leonard P. Hornak for Refueling Machine for a Nuclear Reactor and assigned to Westinghouse Electric Corporation (herein Kowalski), discloses apparatus for transferring fuel assemblies during a refueling operation. The apparatus disclosed in Kowalski operates entirely satisfactorily. However, during a refueling operation it is frequently necessary to transfer other component assemblies of a reactor than the fuel assemblies. Among these assemblies are the control-rod assemblies or clusters and thimble-plug assemblies. Preparatory to a refueling operation the control rods are inserted in the fuel assemblies to set the reactor in a non-critical state. A solution of a material, such as a compound of boron.sup.10, having a high neutron absorption cross section is then injected into the coolant. The control rods may now be removed from the fuel assemblies when it is desirable that the control-rod assembly be removed from a fuel assembly that is to be transferred. In some cases the control-rod assembly is transferred to a different position than the fuel assembly. Some fuel assemblies do not contain fuel rods. Instead the control-rod thimbles are closed by plugs so that the coolant flow is maintained uniform through all fuel assemblies. It is sometimes necessary that the plug assemblies be transferred. It is then necessary that apparatus be provided for transferring, not only the fuel assemblies, but also other component assemblies, such as those mentioned above.
In accordance with the teachings of the prior art (for example, Kuhn U.S. Pat. No. 4,279,699, Kruger U.S. Pat. No. 3,691,011) apparatus for transferring both fuel assemblies and other component assemblies includes within a guiding mast, an inner mast for transferring fuel assemblies within which there is an elongated member for transferring other component assemblies. The elongated member is driven upwardly and downwardly by a winch or the like. To selectively drive the inner mast, the elongated member is provided with latches. The latches are actuable to be engaged with cooperative members on the inner mast when the inner mast is to be raised. This occurs when a fuel assembly is to be transferred. When only the elongated member is to be raised, the latches are set to remain disengaged from the cooperative members on the inner mast.
This prior art structure demands that a plurality of latches be movably mounted on the longitudinal member. The latches are dimensioned to be accommodated on the longitudinal member within the region between the longitudinal member and the inner mast. Difficulty and high cost has been experienced in the fabrication, assembly on the longitudinal member, and maintenance of the latches. The weight of these latches has also been found to be a negative factor. Notwithstanding that the thickness of these latches is substantial, it has been found that they must be constructed of exotic material, for example, age-hardened stainless steel.
Another problem presented by prior art involves the stability of the inner mast as it is raised or lowered. This involves the cooperative functioning of the guiding mast and the inner mast. Hoffmeister U.S. Pat. No. 3,978,957, which is typical of the prior art, discloses fuel-assembly transfer apparatus in which the mast which raises the fuel assembly is of rectangular cross section and is guided by rollers along a guiding mast. At each guiding position, pairs of rollers engage plates along opposite corners of the rectangular mast. This structure is costly, complicated and difficult to install and maintain. At each guiding position a pair of rollers must be mounted and maintained so that their guiding operation is appropriately coordinated. The guiding function is not positive since the rollers at a corner may become displaced with reference to each other.
It is an object of this invention to overcome the above-described disadvantages and drawbacks of the prior art and to provide apparatus for transferring both fuel assemblies and other component assemblies of a reactor during refueling, the coordinated movement of whose raising mechanisms shall be carried out with parts composed of non-exotic, relatively low-cost materials, whose parts are readily fabricated and assembled and which are readily maintainable. It is also an object of this invention to provide such apparatus including guiding means, of simple structure which shall be readily installed and maintained, for the inner mast which carries the fuel assemblies, which guiding means shall be of simple structure and which shall be composed of a minimum of relatively low-cost parts.